U.S. patent number 9,784,558 [Application Number 14/588,942] was granted by the patent office on 2017-10-10 for sensing of mirror position using fringing fields.
This patent grant is currently assigned to APPLE INC.. The grantee listed for this patent is APPLE INC.. Invention is credited to Raviv Erlich.
United States Patent |
9,784,558 |
Erlich |
October 10, 2017 |
Sensing of mirror position using fringing fields
Abstract
Mechanical apparatus includes a base and a moving element, which
is mounted to rotate about an axis relative to the base. A
capacitive rotation sensor includes at least one first electrode
disposed on the moving element in a location adjacent to the base
and at least one second electrode disposed on the base in proximity
to the at least one first electrode. A sensing circuit is coupled
to sense a variable capacitance between the first and second
electrodes.
Inventors: |
Erlich; Raviv (Rehovot,
IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
APPLE INC. |
Cupertino |
CA |
US |
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Assignee: |
APPLE INC. (Cupertino,
CA)
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Family
ID: |
52350397 |
Appl.
No.: |
14/588,942 |
Filed: |
January 4, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150204650 A1 |
Jul 23, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61929140 |
Jan 20, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
26/0833 (20130101); G02B 26/10 (20130101); G01B
7/30 (20130101); G02B 26/085 (20130101) |
Current International
Class: |
G01R
27/26 (20060101); G02B 26/10 (20060101); G02B
26/08 (20060101); G01B 7/30 (20060101) |
Field of
Search: |
;324/686,207.24,173,174,200,207.11,207.25 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
International Application # PCT/US15/10078 Search Report dated Mar.
23, 2015. cited by applicant .
Hofmann et al., "MEMS scanning laser projection based on high-q
vacuum packaged 2D-resonators", Moems and Miniaturized Systems X,
Proceedings of SPIE, vol. 7930, No. 1, pp. 1-10, Feb. 10, 2011.
cited by applicant.
|
Primary Examiner: Astacio-Oquendo; Giovanni
Attorney, Agent or Firm: D. Kligler IP Services Ltd.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent
Application 61/929,140, filed Jan. 20, 2014, which is incorporated
herein by reference.
Claims
The invention claimed is:
1. Mechanical apparatus, comprising: a base; a moving element,
which is mounted to rotate about an axis relative to the base; and
a capacitive rotation sensor, comprising: electrodes, including a
first electrode disposed on the moving element in a location
adjacent to the base and a second electrode disposed on the base in
proximity to the first electrode; and a sensing circuit, which is
coupled to sense a variable capacitance between the first electrode
and the second electrode, which capacitance varies nonlinearly with
an angle of rotation of the moving element, and to measure both a
magnitude of the capacitance and a slope of variation of the
capacitance with rotation of the moving element, and to apply both
the measured magnitude and the measured slope in finding and
outputting the angle of rotation as a function of time.
2. The apparatus according to claim 1, wherein the base defines a
plane, and the moving element has a mechanical equilibrium position
in the plane, such that the electrodes are coplanar when the moving
element is in the mechanical equilibrium position.
3. The apparatus according to claim 1, wherein the base and the
moving element are formed from a semiconductor substrate in a
microelectromechanical systems (MEMS) process, and wherein the
electrodes and conductive traces connecting the electrodes to the
sensing circuit are deposited on the semiconductor substrate as a
part of the MEMS process.
4. The apparatus according to claim 1, wherein the moving element
comprises a gimbal, and the base comprises a frame on which the
gimbal is mounted.
5. The apparatus according to claim 1, wherein the moving element
comprises a mirror, and the base comprises a gimbal on which the
mirror is mounted.
6. The apparatus according to claim 1, wherein the first electrode
and the second electrode have respective shapes that are elongated
along a direction perpendicular to the axis about which the moving
element rotates.
7. The apparatus according to claim 1, wherein the first electrode
and the second electrode have respective shapes that are elongated
along a direction parallel to the axis about which the moving
element rotates.
8. The apparatus according to claim 1, wherein the sensing circuit
is configured to make an absolute measurement of an angular
position of the moving element based on a location of a central
peak in a curve corresponding to the variation of the capacitance
with rotation of the moving element and a shape of the curve.
9. A method for sensing, comprising: mounting a moving element to
rotate about an axis relative to a base; disposing electrodes on
the moving element and the base, including a first electrode
disposed on the moving element in a location adjacent to the base
and a second electrode disposed on the base in proximity to the
first electrode; sensing, using a sensing circuit, a variable
capacitance between the first electrode and the second electrode,
which capacitance varies nonlinearly with an angle of rotation of
the moving element, as the moving element rotates about the axis;
measuring both a magnitude of the capacitance and a slope of
variation of the capacitance with rotation of the moving element;
and applying both the measured magnitude and the measured slope in
finding and outputting the angle of rotation as a function of
time.
10. The method according to claim 9, wherein the base defines a
plane, and the moving element has a mechanical equilibrium position
in the plane, such that the electrodes are coplanar when the moving
element is in the mechanical equilibrium position.
11. The method according to claim 9, wherein mounting the moving
element comprises forming the base and the moving element from a
semiconductor substrate in a microelectromechanical systems (MEMS)
process, and wherein disposing the electrodes comprises depositing
the electrodes and conductive traces connected to the electrodes on
the semiconductor substrate as a part of the MEMS process.
12. The method according to claim 9, wherein the moving element
comprises a gimbal, and the base comprises a frame on which the
gimbal is mounted.
13. The method according to claim 9, wherein the moving element
comprises a mirror, and the base comprises a gimbal on which the
mirror is mounted.
14. The method according to claim 9, wherein the electrodes have
respective shapes that are elongated along a direction
perpendicular to the axis about which the moving element
rotates.
15. The method according to claim 9, wherein the electrodes have
respective shapes that are elongated along a direction parallel to
the axis about which the moving element rotates.
16. The method according to claim 9, wherein finding and outputting
the angle of rotation comprises making an absolute measurement of
an angular position of the moving element based on a location of a
central peak in a curve corresponding to the variation of the
capacitance with rotation of the moving element and a shape of the
curve.
Description
FIELD OF THE INVENTION
The present invention relates to monitoring the motion of rotating
mechanical devices, and particularly of scanning micromirrors.
BACKGROUND
PCT International Publication WO 2014/016794, whose disclosure is
incorporated herein by reference, describes scanning micromirrors,
which are based on microelectromechanical systems (MEMS).
Embodiments described in this publication provide scanning mirror
assemblies that include a support structure; a base (also referred
to as a gimbal), which is mounted to rotate about a first axis
relative to the support structure; and a mirror, which is mounted
to rotate about a second axis relative to the base.
In one of the embodiments described in WO 2014/016794, capacitive
sensing is used to monitor the rotation of the mirror, using plates
of a capacitive sensor that are positioned in proximity to the
mirror on opposite sides of the axis of rotation. In the disclosed
embodiment, the plates are angled relative to the plane of the
support structure, although in other implementations, the plates
may be parallel to the plane of the support structure. Changes in
the capacitance between the plates and the mirror are measured so
as to monitor rotation of the mirror.
SUMMARY
Embodiments of the present invention that are described hereinbelow
provide improved techniques for capacitive sensing of miniature
moving elements.
There is therefore provided, in accordance with an embodiment of
the present invention, mechanical apparatus, which includes a base
and a moving element, which is mounted to rotate about an axis
relative to the base. A capacitive rotation sensor includes at
least one first electrode disposed on the moving element in a
location adjacent to the base and at least one second electrode
disposed on the base in proximity to the at least one first
electrode. A sensing circuit is coupled to sense a variable
capacitance between the first and second electrodes.
In disclosed embodiments, the base defines a plane, and the moving
element has a mechanical equilibrium position in the plane, such
that the first and second electrodes are coplanar when the moving
element is in the mechanical equilibrium position. The base and the
moving element may be formed from a semiconductor substrate in a
microelectromechanical systems (MEMS) process, wherein the
electrodes and conductive traces connecting the electrodes to the
sensing circuit are deposited on the semiconductor substrate as a
part of the MEMS process.
In one embodiment, the moving element includes a gimbal, and the
base includes a frame on which the gimbal is mounted. Additionally
or alternatively, the moving element may include a mirror, while
the base includes a gimbal on which the mirror is mounted.
Typically, the first and second electrodes have respective shapes
that are elongated along a direction perpendicular and/or parallel
to the axis about which the moving element rotates.
In the disclosed embodiments, the sensing circuit is configured to
output, responsively to the sensed capacitance, an indication of an
angle of rotation of the moving element relative to the base. In
some embodiments, the capacitance sensed by the sensing circuit
varies nonlinearly with the angle of rotation of the moving
element, and the sensing circuit is configured to apply both a
magnitude of the capacitance and a slope of variation of the
capacitance with rotation of the moving element in finding the
angle of rotation as a function of the sensed capacitance.
There is also provided, in accordance with an embodiment of the
present invention, a method for sensing, which includes mounting a
moving element to rotate about an axis relative to the base. At
least one first electrode is disposed on the moving element in a
location adjacent to the base, and at least one second electrode is
disposed on the base in proximity to the at least one first
electrode. A variable capacitance is sensed between the first and
second electrodes as the moving element rotates about the axis.
The present invention will be more fully understood from the
following detailed description of the embodiments thereof, taken
together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic frontal view of a scanning assembly
comprising a gimbaled scanning mirror with a capacitive rotation
sensor, in accordance with an embodiment of the present
invention;
FIG. 2 is a schematic frontal view of a scanning assembly
comprising a gimbaled scanning mirror with a capacitive rotation
sensor, in accordance with another embodiment of the present
invention;
FIG. 3 is a plot showing calculated capacitance curves as a
function of rotation angle of a gimbal, in accordance with an
embodiment of the present invention;
FIG. 4 is a block diagram that schematically illustrates a scanning
system, in accordance with an embodiment of the present invention;
and
FIG. 5 is a flow chart that schematically illustrates a method for
scanning, in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS
Embodiments of the present invention that are described hereinbelow
provide capacitive sensing of the rotation of a moving element
relative to a base. In the disclosed embodiments, the moving
element is a gimbal, which holds a scanning mirror, while the base
is a frame on which the gimbal rotates. Alternatively or
additionally, the mirror may be the moving element, while the
gimbal is the base. Further alternatively, the principles of the
disclosed embodiments may be applied to devices of other types that
include moving elements, particularly planar devices, such as MEMS
devices.
The disclosed embodiments sense rotation of the moving element
without electrodes outside the device plane, which in typical MEMS
implementations is the plane of the wafer. Thus, in the case of a
scanning mirror or gimbal, for example, sensing of rotation is
accomplished using electrodes that have been formed only in the
plane of the mirror structure. These embodiments use changes in the
capacitance between a pair of electrodes that are positioned side
by side in-plane on the mirror structures themselves. The
capacitance in this case changes as the result of changes of the
fringing electric fields with distance between the electrodes and
thus provides a means for accurately monitoring rotation angle.
This sensing approach not only enables accurate measurement, but is
also inexpensive and simple to implement. Because the capacitance
varies nonlinearly with angle, it can be used for absolute position
measurement, based on the location of the peak in the capacitance
curve (which typically corresponds to the in-plane, zero-torque
angle of the rotating device), as well as the shape of the curve.
This mode of measurement is thus resilient in the face of gain
variations of amplifiers in the sensing circuit and other factors
that could otherwise distort the scale of the capacitance
measurement.
In the embodiments that are shown in the figures and are described
in detail hereinbelow, capacitive sensors of this sort are used in
sensing relative motion between a rotating gimbal and a frame,
which serves as the support structure for the gimbal and mirror. In
alternative embodiments, not shown in the figures, capacitive
sensors based on fringing electric fields may be used, additionally
or alternatively, in measuring the rotation angle of the mirror
relative to the gimbal. More generally, the principles of the
present invention may be applied in monitoring rotating structures
of other types, particularly in MEMS devices, in which sensors of
this sort can be produced as part of the photolithographic
manufacturing process that is used in fabricating the devices
themselves.
FIG. 1 is a schematic frontal view of a scanning assembly 20, which
comprises a gimbaled scanning mirror 26 with a capacitive rotation
sensor, in accordance with an embodiment of the present invention.
Mirror 26 rotates on a pair of torsion hinges 30 (oriented along
the Y-axis in the figure) relative to a gimbal 24, which in turn
rotates on another pair of torsion hinges 28 (oriented along the
X-axis) relative to a frame 22. Rotation of mirror 26 and gimbal 24
may be driven, for example, by the sorts of magnetic drives that
are described in the above-mentioned PCT publication, or by any
other suitable sort of drive that is known in the art.
Scanning assembly 20 may typically be produced from a semiconductor
wafer by MEMS micro-fabrication processes, in which the borders of
mirror 26, gimbal 24 and hinges 28, 30 are defined by a
photolithographic mask, and the wafer is then etched to release the
moving mirror and gimbal from the surrounding parts of the wafer.
As another step in this process, a reflective metal coating (not
shown) is deposited on the surface of the mirror. In this same
step, or in another metal deposition step, inner metal electrodes
34 and 38 are deposited along the edges of gimbal 24, and outer
metal electrodes 32 and 36 are deposited on an adjacent area of
frame 22, as shown in the figure. Conductive traces 40, 44, 46 are
also deposited on the wafer surface, connecting electrodes 32, 34,
36, 38 to connection pads 48. It may be desirable to deposit an
insulating layer, such as an oxide layer, over the wafer before
depositing the metal electrodes, in order to eliminate any possible
ohmic coupling between electrodes 34 and 38 on gimbal 24 and
electrodes 32 and 36 on frame 22.
Each pair of metal electrodes--one electrode 34 or 38 on gimbal 24
and the other electrode 32 or 36 on frame 22--define a capacitor.
The capacitance between the electrodes in each pair, due to the
fringing fields of the electrodes, varies as a function of the gap
between the electrodes and thus changes with the tilt angle of the
gimbal. Frame 22 defines a plane, identified for convenience as the
X-Y plane in FIG. 1. Gimbal 24 (as well as mirror 26) has a
mechanical equilibrium position (zero torque angle) in the plane,
such that electrodes 32, 34, 36 and 38 are coplanar when the gimbal
is in the mechanical equilibrium position. Electrodes 23, 24, 26
and 38 have elongated shapes, with the long axes of the pairs of
electrodes 32/34 and 36/38 oriented in the Y-direction,
perpendicular to the axis of hinges 28 about which the rotation of
gimbal 24 is to be measured.
To measure the capacitance, and thus the angle of rotation (also
referred to as the tile angle) of gimbal 24, a sensing circuit 50
is connected to contact pads 48 and senses the variable impedance
between electrodes 32 and 34 and between electrodes 36 and 38.
Sensing circuit 50 may sense the impedance, for example, by
applying a modulated voltage between the electrodes, via conductive
traces 40 44 and 46, and sensing the resulting current (or vice
versa). Sensing circuit 50 converts the sensed impedance to a
corresponding value of rotation angle, typically based on a
calibration function that is determined in advance. For these
purposes, sensing circuit 50 may comprise, for example, a digital
logic circuit with a frequency synthesizer and suitable
digital/analog and analog/digital converters for analog coupling to
the electrodes of assembly 20, as well as a digital output, which
outputs an indication of the angle of rotation.
In typical applications, sensing circuit 50 outputs this indication
of the rotation angle to a system controller (not shown in the
figures), which may use the angular value, for example, in
closed-loop control of the rotation of assembly 20. Additionally or
alternatively, the system controller may apply the angle
measurements provided by sensing circuit 50 in calibrating and
controlling the operation of a system based on scanning assembly
20, such as a scanning LIDAR or projection system. Details of a
system of this sort are shown in FIG. 4, while methods of control
and calibration in such a system are shown in FIG. 5 and are
described hereinbelow with reference to these figures.
FIG. 2 is a schematic frontal view of a scanning assembly 60, in
accordance with an alternative embodiment of the present invention.
Assembly 60 is mechanically substantially identical to assembly 20
(FIG. 1), but in the present embodiment, the metal pads that serve
as electrodes 62, 64 and 66 of the capacitive rotation sensor are
located in different areas of gimbal 24 and frame 22. The long axes
of electrode pairs 62/66 and 64/66 in assembly 60 are oriented
along the X-direction, parallel to the axis of hinges 28. In other
respects, the operation of the capacitive rotation sensor in FIG. 2
is similar to that in the preceding embodiment. The electrode
configuration of FIG. 1 is particularly effective for measuring
rotation angle, while that of FIG. 2 provides precise sensing of
the in-plane, zero-torque position of the gimbal. In practice, the
two embodiments may advantageously be combined, with electrodes
deployed both perpendicular (as in FIG. 1) and parallel (as in FIG.
2) to the axis of hinges 28 about which gimbal 24 rotates.
As noted earlier, the rotation of mirror 26 relative to gimbal 24
in assembly 60 can be monitored in similar fashion, by depositing
electrodes on the mirror and on adjacent areas of the gimbal. Since
the mirror has a reflective metal coating anyway, this metal
coating may optionally also serve as an electrode of the capacitive
sensor.
FIG. 3 is a plot showing calculated capacitance curves 80, 82, 84,
86, 88 as a function of rotation angle of gimbal 24 relative to
frame 22, in accordance with an embodiment of the present
invention. The calculation is based on a configuration that
combines the electrodes of FIGS. 1 and 2, for different lengths L
of the side electrodes (32/34 and 36/38). The lengths are smallest
in curve 80 and increase in steps up to curve 88, which represents
the capacitance using the longest electrodes. As illustrated in
FIG. 3, although FIGS. 1 and 2 show particular electrode shapes and
sizes, these features of the electrodes can readily be modified to
give the desired capacitance range and behavior of the sensor.
As shown by the curves in FIG. 3, the variation of capacitance is
not linear in angle. Consequently, both the magnitude of the
capacitance and the local slope of the curve can be used in
measuring the rotation angle, and the accuracy of measurement can
thus be enhanced. Because the two sets of electrodes--those on
gimbal 26 and those on frame 24--are formed on the same wafer, any
temperature variations will have the substantially same effect on
both sets of electrodes and thus will have no more than minimal
impact on the measurement accuracy.
Moreover, the nonlinearity of the variation of capacitance with
angle can be used for absolute position measurement, based on the
location of the central peak (corresponding to the in-plane,
zero-torque angle) and the shape of the curve. This mode of
measurement is thus resilient in the face of gain variations of the
amplifiers and other factors that could otherwise distort the scale
of the capacitance measurement. Compensating for such factors in a
linear sensing configuration can require a difficult calibration
procedure.
Furthermore, although FIGS. 1 and 2 show certain particular
arrangements of the capacitive sensing electrodes on frame 22 and
gimbal 24, any other suitable arrangement of one or more pairs of
electrodes may be used for this purpose, so long as the sizes of
and spacing between the electrodes are such as to give a
substantial capacitive response that varies with rotation of the
gimbal or other structure.
FIG. 4 is a block diagram that schematically illustrates a scanning
system 100, in accordance with an embodiment of the present
invention. System 100 comprises an optical head 102, which
incorporates scanning assembly 20 and sensing circuit 50, as
described above. An optical transmitter/receiver 104 transmits
pulses of light toward mirror 26 in scanning assembly 20 and
receives light returned from the mirror. Alternatively, optical
head 102 may comprise only the transmitter or only the receiver.
Driver circuits 106 control the scanning frequency, phase and
amplitude of scanning assembly 20, as well as controlling operation
of transmitter/receiver 104, such as the amplitude and repetition
rate of the transmitted pulses.
A controller 108 comprises control circuits 110, which receive
signals from sensing circuit 50 and provide control outputs
accordingly to drivers 106 under the command of a system processor
112, which comprises one or more processing units. The control
outputs may, for example, cause drivers 106 to adjust the
frequency, phase and/or amplitude of scanning assembly 20 as
necessary. Processor 112 may also use the readings of scanning
angle provided by sensing circuit in processing the signals output
by the receiver in optical head 102. Controller 108 typically
comprises ancillary circuits, such as a power supply 114 and other
components that are known in the art. Although the functional
elements of controller 108 are shown in FIG. 4, for the sake of
conceptual clarity, as separate blocks, some or all of these
elements may be combined in a single integrated circuit.
FIG. 5 is a flow chart that schematically illustrates a method for
scanning using system 100, in accordance with an embodiment of the
present invention. Controller 108 receives a sequence of input
signals or data from sensing circuit 50, indicating the rotation
angle of scanning assembly 20 as a function of time, at a sense
input step 120. Based on these signals or data, the controller
computes the actual rotation angle as a function of time, at an
angle computation step 122. The computed angles may be used for (at
least) two purposes: Based on the angle readings, controller 108
computes the frequency of rotation of scanning assembly 20, as well
as the phase and amplitude of rotation, at a frequency computation
step 124. The controller checks these values against corresponding
benchmarks, such as preset frequency and amplitude targets, at a
parameter checking step 126. If the computed values deviate from
the benchmarks, controller 108 sends an appropriate command to
driver circuits 106, so as to cause the driver to adjust the
scanning parameters. The control loop (regardless of the result of
step 126) then returns to step 120 for the next iteration.
Controller 108 may also use the angle readings in calibrating the
signals received from transmitter/receiver 104 in optical head 102,
at a signal calibration step 130. For example, the angle readings
may be used in order to ascertain accurately the angle at which
each signal from the receiver is received.
It will thus be appreciated that the embodiments described above
are cited by way of example, and that the present invention is not
limited to what has been particularly shown and described
hereinabove. Rather, the scope of the present invention includes
both combinations and subcombinations of the various features
described hereinabove, as well as variations and modifications
thereof which would occur to persons skilled in the art upon
reading the foregoing description and which are not disclosed in
the prior art.
* * * * *